1. Field of the Invention
The present invention relates to an electrolytic solution supply type battery in which a plurality of unit cells are electrically series-connected or stacked and, more particularly, to a battery of this type wherein liquid short-circuit among unit cells is prevented.
So-called electrolytic solution supply type batteries wherein an electrolytic solution is externally supplied and is exhausted outside the cells through an electrode reaction section within each cell include several types such as zinc-halogen batteries, redox cells and fuel cells. In order to increase the capacity of such a battery, a number of unit cells must be connected in series or parallel Since series connection allows adoption of a bipolar electrode structure, it is very advantageous for increasing the capacity of a battery.
2. Description of the Prior Art
When unit cells are series-connected or stacked in such an electrolytic solution supply type battery, the circulation method is generally adopted. In this method, an electrolytic solution is generally supplied from a common electrolytic solution tank, and the electrolytic solution exhausted from each unit cell is recovered in the electrolytic solution tank. FIG. 3 shows an example of such a battery. Referring to FIG. 3, four unit cells C-1, C-2, C-3 and C-4 are series-connected. An electrolytic solution 3 is respectively supplied from an electrolytic solution tank T to the unit cells C-1, C-2, C-3 and C-4 by a pump P through a common supply path 1' and respective distribution liquid paths 2'a, 2'b, 2'c and 2'd. The electrolytic solution exhausted from the respective unit cells is returned to the tank T through respective exhaust liquid paths 4'a, 4'b, 4'c and 4'd and a common exhaust path 5'. However, when the battery of this arrangement is operated, as shown in FIG. 4, in addition to a current I of the battery, a liquid short-circuit current (shunt current) I' flows among the cells through the electrolytic solution in the distribution and exhaust liquid paths. This causes a liquid short-circuit and results in a large current loss. When this state is represented by an electric equivalent circuit, it is as shown in FIG. 5. Referring to FIG. 5, reference symbols R1 to R4 represent resistors.
When such a liquid short-circuit occurs, the discharge capacity is decreased in the case of a primary battery. In the case of a secondary battery, both the charge and discharge capacities are decreased and charge and discharge efficiencies are considerably decreased. In order to prevent such a liquid short-circuit and its adverse influence, the resistances of the electrolytic solution portions in the liquid path between each two adjacent unit cells, i.e., R1 to R4 in FIG. 5 are increased. More specifically, the lengths of the liquid paths between each two adjacent unit cells (e.g., l1, l2, W1 and W2 in FIG. 4) are increased or the cross-sectional areas of the liquid paths (e.g. S1 to S4 in FIG. 4) are decreased in accordance with the equation R=.rho..multidot.l/S (where R: resistance, .rho.: resistivity; l: length of liquid path; S: cross-sectional area). In accordance with another method, a rotating member is arranged in a liquid path to cut off the electrolytic solution and to interrupt the continuity of the electrolytic solution.
However, when the liquid path length between each two adjacent cells is increased, the piping is increased. This renders the battery structure complex and increases the battery volume. Furthermore, when the cross-sectional area of the liquid path is decreased, the resistance of the electrolytic solution is increased, and pressure loss is increased. In either case, specific problems are encountered and liquid short-circuit cannot be completely prevented. With the method in which the electrolytic solution is cut off by using a rotating member, the piping is rendered complex, and mechanical durability of the rotating member presents a problem, resulting in an unsatisfactory resolution of the problem.